1. Field of the Invention
The present invention relates to a color image forming method and apparatus for sequentially forming toner images of a plurality of colors on a photosensitive body so as to form a color toner image.
2. Description of the Prior Art
In an image forming apparatus based on electrophotography or electrostatic recording, an electrostatic latent image is formed on a photosensitive body, and is developed by a toner as charged particles. A color image and a composite image (an overlap of a plurality of documents or an overlap of image data and a document image) are obtained by utilizing the above-mentioned principle in the following manner. The cycle of charging, image exposure, and developing is repeated twice or more on a photosensitive body having a photoconductive layer on a conductive substrate (e.g., Japanese Patent Application No. 53-184381). In another method, a photosensitive body having a transparent insulating layer formed outside a photoconductive layer is used, and the cycle of primary charging, secondary charging, image exposure, uniform exposure, and developing is repeated twice or more. In still another method, the cycle of primary charging, secondary charging, image exposure, and developing is performed twice or more (e.g., Japanese Patent Application No. 58-183152). In each image forming method described above, color developing and overlapping of images can be performed on the photosensitive body. Since these overlapping images can be transferred onto a transfer medium by one transfer process, a color image and a composite image can be obtained by an apparatus having a simple arrangement.
The above-described image forming process is realized by a developing method in which, for example, a developing agent consisting of a nonmagnetic toner and a magnetic carrier is used and developing is performed under conditions described in Japanese Patent Application No. 58-57446 or 60-192712. In a developing unit, the developing agent is agitated, and the toner is negatively charged. The charged toner is then attracted to the magnetic carrier surface by static electricity. The developing agent in which the toner is electrostatically coupled to the carrier is magnetically attracted to a developing sleeve surface and is transferred to a developing region while it is rotated at a predetermined linear velocity. This developing method is a kind of magnetic brush developing method, and is characterized in that only the toner is caused to fly to a latent image surface of the photosensitive body by an AC bias without bringing the magnetic brush into contact with the photosensitive body.
According to an image forming apparatus suitable for the above-described image forming process and developing method, latent images are formed by a latent image forming means in units of colors, and developing is performed by developing units using toners of colors corresponding to the respective latent images.
In a typical image forming apparatus of this type, an electrostatic latent image is formed by radiating a light beam such as a laser beam on a photosensitive body having a photoconductive substance on a conductive substrate.
FIG. 1 shows a schematic arrangement of a conventional color image forming apparatus of this type.
Referring to FIG. 1, reference numeral 1 denotes a drum-like photosensitive body which has a photoconductive layer on a conductive substrate and is rotated at a predetermined angular velocity in a direction indicated by an arrow; 2, an image exposure unit for performing image exposure of the uniformly-charged surface of the photosensitive body 1; 3, a scorotron charger for uniformly charging the surface of the photosensitive body 1; 4a, 4b, 4c, and 4d, developing units respectively storing yellow, magenta, cyan, and black toners together with a nonmagnetic carrier; 5, a paper feed tray on which transfer sheets P are placed; 6, a pre-transfer exposure lamp for exposing the surface of the photosensitive body 1 on which a color toner image or a composite toner image is formed; 7, a transfer unit for transferring the toner image onto the transfer sheet P upon discharging; 8, a separator for separating the transfer sheet P from the surface of the photosensitive body 1; 9, a conveyor belt for conveying the transfer sheet P, onto which the toner image was transferred, to a fixing unit 10 upon separation; 10, the fixing unit for fixing the toner image on the transfer sheet; 11, a cleaning discharger for discharging the photosensitive body 1 upon transfer; and 12, a cleaning unit for removing the residual toner on the surface of the photosensitive body 1 by using a blade 12a, a bias roller 12b, and a cleaning roller 12c which have been set in an inoperative state during toner image formation, thus making the apparatus ready for the next color image formation.
Each of the developing units 4a to 4d includes the following components in its developing tank: first and second agitating members, a feed roller, a scraper, a thin-layer forming plate, and a developing agent carrier. The developing agent carrier is arranged near the photosensitive body 1. In order to prevent fogging, the apparatus includes a developing bias circuit for applying a bias voltage to the sleeve through a protective resistor. The developing bias circuit includes an AC power source for supplying an AC bias to cause oscillation between the sleeve and the photosensitive body in a developing region, and a high-voltage DC power source for supplying a DC bias. With this arrangement, the developing bias circuit generates an oscillating electric field between the sleeve and the photosensitive body. Since developing agent particles oscillate between the sleeve and the photosensitive body, a toner image can be formed on the photosensitive body by toner particles without much contact between the developing agent and the photosensitive body.
FIG. 2 shows a detailed arrangement of the image exposure unit 2 (enclosed with a dotted line).
Referring to FIG. 2, reference numeral 21 denotes a frame memory. Yellow, magenta, cyan, and black image data Y, M, C, and BK each corresponding to one frame are written in the frame memory 21. In this case, for example, a packed pixel format shown in FIG. 3A or a planar pixel format shown in FIG. 3B is available as a recording format. In the packed pixel format, the image data Y, M, C, and BK are stored in units of pixels. In the planar pixel format, the image data Y, M, C, and BK are respectively stored in different regions.
As described above, in the step of forming a yellow toner image, the yellow data Y are sequentially read out from the frame memory 21 and are supplied to a laser driver 22. A semiconductor laser 23 is driven on the basis of the yellow image data Y. Image exposure light L output from the semiconductor laser 23 is radiated onto the photosensitive body 1, which is uniformly changed in advance, through a rotating polygon mirror 24 for deflection and an f-.theta. lens 25 for focusing. As a result, a latent image corresponding to the yellow image data Y is formed on the photosensitive body 1. This image is then developed by the developing unit A to form a yellow toner image.
In the step of forming a magenta toner image, the magenta data M are sequentially read out from the frame memory 21 and are supplied to the laser driver 22. The semiconductor laser 23 is driven on the basis of the magenta image data M. The image exposure light L output from the semiconductor laser 23 is radiated onto the photosensitive body 1, which is uniformly changed in advance, through the rotating polygon mirror 24 for deflection and the f-.theta. lens 25 for focusing. As a result, a latent image corresponding to the magenta image data M is formed on the photosensitive body 1. This image is then developed by the developing unit B to form a magenta toner image.
Subsequently, the same operation as described is performed in the steps of forming cyan and black toner images. As a result, latent images respectively corresponding to the cyan and black image data C and BK are formed on the photosensitive body 1. These images are then developed by the developing units C and D, respectively, and cyan and black toner images are formed.
A color toner image formed on the photosensitive body 1 will be considered.
A color image is formed in a toner image forming process shown in FIGS. 4(A) to 4(G).
FIGS. 4(A) to 4(F) are views for explaining the color image forming process in which toner images are caused to overlap on a photosensitive body by repeatedly performing image exposure and developing. In this case, the photosensitive body is a drum-like organic photoconductor obtained by forming a charge generating layer (to be referred to as a CGL hereinafter) on a conductive substrate consisting of Al (aluminum) or the like. The substrate is grounded. The CTL and CGL are constituted by dielectric materials.
As shown in FIG. 4(A), charges (negative charges in this case) are uniformly distributed on the surface of the photosensitive body upon scorotron discharging by means of a scorotron discharger in order to set a uniform surface potential. With this operation, the CTL and the CGL as dielectric materials of the photosensitive body are subjected to dielectric polarization.
Subsequently, an electrostatic latent image is formed by radiating a laser beam from the laser exposure unit. Upon radiation of the laser beam, as shown in FIG. 4(B), charges are generated in the CGL, and the energy in the CTL is enhanced by the laser beam. As a result, holes as positive charges generated in the CGL are moved and attracted to negative charges on the photosensitive body surface so as to be electrically neutralized. At this time, the potential of the photosensitive body surface on which the electrostatic latent image is formed is changed to an exposure potential V.sub.L corresponding to the amount of laser beam. As a result, a developing potential gap V.sub.G is generated, which corresponds to a potential difference between the exposure potential V.sub.L and a surface potential V.sub.DC of the developing sleeve based on a DC bias supplied from a developing bias circuit of a developing unit. Since an electric flux generated by this developing potential gap V.sub.G flows from the latent image surface of the photosensitive body surface to the developing sleeve surface, toner particles (e.g., yellow toner particles) as negatively charged particles are attracted by the electrical force directed to the latent image portion of the photosensitive body surface. However, the above-mentioned force is not large enough to separate the toner particles as charged particles from the magnetic carrier, to which the toner particles are coupled by static electricity. When an oscillating electric field is generated between the developing sleeve and the photosensitive body by applying an AC bias from the developing bias circuit, particles of the developing agent carried on the developing sleeve surface by a magnetic force oscillate between the sleeve and the photosensitive body. As a result, the coupling force of the toner as the charged particles and the magnetic carrier is weakened, and the toner particles as the negatively charged particles fly to the electrostatic latent image formed on the photosensitive body surface owing to an electrical force, and are electrostatically attracted to the electrostatic latent image. In this manner, as shown in FIG. 4(C), the toner particles (yellow) as negatively charged particles are electrostatically attracted to the electrostatic latent image on the photosensitive body so as to perform developing, thus forming a first toner (yellow) image.
As shown in FIG. 4(D), for the next latent image forming operation, the surface of the photosensitive body on which the toner (yellow) layer is formed by the above-described first developing operation is re-charged by uniformly distributing negative charges on the surface upon scorotron discharging by the charger.
The second image exposure light is radiated from the laser exposure unit so as to form the second latent image on the photosensitive body surface. More specifically, as shown in FIG. 4(E), charges are generated in the CGL upon radiation of the laser beam, and the energy in the CTL is enhanced by the laser beam. As a result, holes as positive charges generated in the CGL are moved and attracted to negative charges on the photosensitive body surface so as to be electrically neutralized. At this time, a developing potential gap V.sub.G is generated, which corresponds to a potential difference between a surface potential V.sub.DC of the developing sleeve and the exposure potential V.sub.L. Since an electric flux generated by this developing potential gap V.sub.G flows from the latent image surface of the photosensitive body surface to the developing sleeve surface, toner particles as negatively charged particles are attracted by the electrical force directed to the latent image portion of the photosensitive body surface. However, the above-mentioned force is not large enough to separate the toner particles as charged particles from the magnetic carrier, to which the toner particles are coupled by static electricity. When an oscillating electric field is generated between the developing sleeve and the photosensitive body by applying an AC bias from the developing bias circuit, particles of the developing agent carried on the developing sleeve surface by a magnetic force oscillate between the sleeve and the photosensitive body. As a result, the coupling force of the toner as the charged particles and the magnetic carrier is weakened, and the toner particles (e.g., magenta) as the negatively charged particles fly to the electrostatic latent image formed on the photosensitive body surface owing to the above-described electrical force, and are electrostatically attracted to the electrostatic latent image. In this manner, as shown in FIG. 4(F), the toner particles (magenta) as negatively charged particles are electrostatically attracted to the electrostatic latent image on the photosensitive body so as to perform developing, thus forming a second toner (magenta) image.
Subsequently, the same process is performed a required number of times in order to obtain a color toner image or a composite image. This image is then transferred onto a transfer member, and is heated or pressurized to fix the image. With this operation, a color image is obtained.
FIG. 5 shows the potential of the photosensitive body surface in the first and second image exposures in the above-described color image forming process.
When the second latent image is formed on the photosensitive body surface by the second image exposure, since the surface potential of the first toner layer is decreased to only a toner layer surface potential V.sub.T2, a developing potential gap V.sub.GN in the second and subsequent image exposures is smaller than a first developing potential gap V.sub.G1.
This is because a toner layer formed on the photosensitive body surface blocks a laser beam to reduce the amount of laser beam radiated on the photosensitive body surface, and the surface potential of the photosensitive body cannot be decreased to an exposure potential V.sub.L1 but is decreased only to a re-exposure potential V.sub.L2. In addition, in the toner layer formed by the first developing operation, the charge amount of the negatively charged particles corresponds to a potential V.sub.T obtained by subtracting the re-exposure potential V.sub.L2 from the toner layer surface potential V.sub.T2. Therefore, the surface potential of the second electrostatic latent image formed on the photosensitive body surface is increased.
Since the re-exposure potential V.sub.L2 on the photosensitive body surface is enhanced to be higher than the first exposure potential V.sub.L1 by a potential corresponding to a potential distribution based on the light-shielding properties of the toner layer and the charge amount of the toner itself as charged particles, the second toner (magenta) layer attached to the photosensitive body surface due to the developing gap G2 as the potential difference between the surface potential V.sub.DC of the developing sleeve and the re-exposure potential V.sub.L2 is thinner than the first toner (yellow) layer. Hence, proper color reproduction cannot be performed. In addition, the above-mentioned tendency becomes more conspicuous with an increase in number of toner layer to be stacked.
Assume that a color toner image is finally formed on the photosensitive body 1 as shown in FIG. 4(F). If this color toner image is transferred onto a transfer sheet P, the lowermost layer on the photosensitive body 1 becomes the uppermost layer on the transfer sheet P, as shown in FIG. 4(G).
As a result, in a region where the first toner (yellow) layer and the second toner (magenta) layer overlap, the first toner layer is located at the upper position.
In a region where the first toner (yellow) layer and the second toner (magenta) layer overlap, red is to be reproduced. However, a corresponding portion on the transfer sheet P as a printout becomes yellowish because the yellow component becomes stronger than the magenta component for the above-described reason.
Such a problem will be posed in any portion in which toner images of a plurality of colors overlap. According to the conventional image forming apparatus, therefore, an image having a proper color tone represented by the image data Y, M, C, and K cannot be formed.
In addition, a television signal is expressed by red, green, and blue image data. However, when an image is to be formed in the above-described manner, yellow, magenta, cyan, and black image data are required. That is, in order to form an image by using a television signal, yellow, magenta, cyan, and black image data must be obtained from red, green, and blue image data. In order to perform such color conversion, a data converter is required.